This invention relates to a multi-stage nanofiltration or reverse osmosis membrane module, to processes for using such a module to filter water or to remove hardness, to processes for cleaning or maintaining the permeability of such a module, and to a small-scale system particularly for use in private homes and small commercial buildings.
Hollow fibre semi-permeable membranes are useful for filtering solids rich fluids. Membranes in the nanofiltration and reverse osmosis ranges may also be useful for separating salts. For example, U.S. Pat. No. 5,152,901 describes a nanofiltration membrane material capable of filtering out suspended solids and large organic molecules and generally rejecting calcium salts while generally permeating sodium salts. U.S. Pat. Nos. 4,812,270 and 5,658,460 also describe membranes useful for rejecting salts. Membranes with similar characteristics, such as Stork Friesland""s NR 015-500, are available on the market.
Membranes as described above may be used in the form of hollow fibres operated in an inside-out flow mode. The hollow fibres are suspended between a pair of opposed tube sheets or headers. The headers maintain a separation between the lumens of the membranes and their outer surfaces. Thus, pressurized feed water can be supplied to the lumens of one end of the membranes, permeate can be collected as it leaves the outer surface of the membranes, and a concentrate or retentate can be extracted from the lumens at the other end of the membranes.
Various characteristics of hollow fibre membranes, however, make them difficult to use in such an inside-out flow mode. For example, the inner diameter of the hollow fibre is small which results in significant pressure and flux reductions towards the outlet end of long hollow fibres. The problem is most significant when the feed pressure is low.
U.S. Pat. No. 5,013,437 describes one method of attempting to reduce the problem of pressure and flux loss in long fibres. In an embodiment of that patent, an inside-out hollow fibre filtration module is split into two stages. The retentate from the first stage becomes the feed for the second stage. The ratio of the surface areas of the first to the second stages is preferably about 1.5:1 to 2.25:1. This helps to increase the pressure and velocity of the retentate from the first stage as it becomes the feed to the second stage such that both stages have more nearly equal pressure drops. The stages are arranged concentrically, however, and permeate, particularly from the second stage, must flow along the outside of the fibres to reach an outlet port. With a reasonable packing density of hollow fibre membranes, the head loss in the permeate flow would be substantial if used to filter liquids. Thus the transmembrane pressure differential across the membranes of the second stage is reduced. It is also difficult to pot fibres in an annular ring as required in the ""437 module.
A similar principle has also been used in large scale systems using spiral wound membranes. A large number of membrane modules are arranged in stages. Each successive stage has fewer modules than the preceding stage and the retentate from preceding stages becomes the feed of the succeeding stages. Such a system is both large and complex and not suited to residential or small commercial systems.
Makers of small scale nanofiltration or reverse osmosis membrane filtration systems typically try to address the problems discussed above by using a single stage filtration module, and recirculating the retentate to the feed inlet to increase the velocity of the feed water and the transmembrane pressure. In such systems, the minimum velocity of the feed/retentate is between about 3-10 ft/s. This technique requires a high rejection membrane, and is operated at a very low per pass recovery. This leads to rapid fouling and either frequent cleaning or replacement of the membranes. Energy costs and pressure required are also high.
Another characteristic of semi-permeable membranes is that their pores become fouled over time particularly including, in the case of membranes used for water softening, because of carbonate scaling. In large scale systems, carbonate scaling may be addressed by partially softening the feed water using resin exchange beds or by adding an anti-scalant to the feed water. Such techniques are generally too complex to be practicable in small scale systems, particularly in private homes.
It is an object of the invention to improve on the prior art. It is another object of the invention to provide a membrane filtration module, particularly one that is useful for small scale filtration or water softening. It is another object of the invention to provide a process to clean or reduce scaling of a membrane module, particularly one used for water softening. It is another object of the invention to provide a small scale filtration or water softening system. These objects are met by the combination of features, steps or both found in the claims. The following summary may not describe all necessary features of the invention which may reside in a sub-combination of the following features or in a combination with features described in other parts of this document.
In various aspects, the invention provides a filtration module having a plurality of hollow fibre nanofiltration or reverse osmosis membranes suspended between a pair of opposed headers. The outer surfaces of the membranes are sealed to the headers while their lumens are open at the distal faces of the headers.
Within the module, the hollow fibre membranes are grouped into a plurality of preceding or succeeding stages (some stages being both preceding and succeeding). The lumens of the hollow fibre membranes are open at first and second ends of the stages. Flow between stages occurs across the distal faces of the headers. A module feed inlet is connected in fluid communication with the first end of a first stage. The remaining stages are connected in series behind the first stage with fluid connections between the second end of each preceding stages and the first end of each directly succeeding stage. A module outlet is connected in fluid communication with the second end of a last stage. A permeate collection plenum surrounds the stages and is in fluid communication with each stage. The surface area of the membranes of each preceding stage is between 1 and 2.5 times the surface area of the membranes of a directly succeeding stage and the surface area of the stages decreases from the first stage to the last stage.
To construct the connections between the stages, a first cap covers the distal face of one header and a second cap covers the distal face of the other header. The permeate plenum includes the space between the proximal faces of the headers and an outer shell. Dividers within one or both of the caps collect groups of the membranes into the stages while leaving open fluid connections between the second end of each preceding stage and the first end of each directly succeeding stage. The module inlet and module retentate outlet, typically provided in the caps, are in fluid communication with the first end of the first stage and the second end of the last stage respectively. Thus feed water enters the first end of the first stage and the portion not permeated exits the second end of the first stage. From there, the second end cap directs the feed/retentate to the first end of the second stage. The water not permeated in the second stage arrives at the first cap. In a two stage device, the water not permeated then leaves the module. In a module with more stages, the first cap redirects the feed/retentate to the first end of another stage and the water not permeated flows to the second cap and so on until the second end of the last stage is reached.
The stages are arranged so that each is adjacent the perimeter of the module and interstage flows are generally parallel to the periphery of the module. For example, the stages may be configured as sectors of a cylinder. In smaller modules, typically about 3xe2x80x3 in diameter or less, the membranes may be separated into stages by a spider in each header. In larger modules, groups of membranes may be potted individually or simultaneously into opposed pairs of collars which may be sector shaped. Once potted into the pairs of collars, the membranes may be coated. The pairs of collars are then glued together to form a pair of headers, which are cylindrical when the collars are sector shaped. The pairs of collars are easier to work with than large cylindrical headers and, in particular, facilitate drying during membrane coating procedures. Dividers to separate stages may be made to correspond with the edges of the collars or with separators inserted into the collars.
In an embodiment, the dividers between stages are fitted with valves and arranged such that when feed water flows into the module in a reverse direction, entering through the module retentate outlet, the dividers re-collect the groups of membranes into second preceding and second succeeding stages having first and second ends. The dividers leave open fluid connections generally parallel to the periphery of the module between the second end of each second preceding stage and the first end of each second succeeding stage. In the re-collection of the membranes, the surface area of the membranes of each second preceding stage is between 1 and 2.5 times the surface area of the membranes of a second directly succeeding stage and the surface area of the stages decreases from the first stage to the last stage. This is achieved by using one way valves opening in a direction such that the grouping and re-grouping of membranes is performed by the action of liquid flowing through the module, ie. opening valves where the pressure differential is in the direction that the valve opens and closing valves where the pressure differential is opposite the direction that the valves open.
Modules as described above are used to filter water and can be used to remove hardness when optionally fitted with hollow fibre membranes adapted to selectively reject hardness causing salts. Water to be filtered flows through the stages in series while a filtered and optionally softened permeate is collected from the outer surfaces of the membranes. The membranes may have a permeability of about 0.1 gfd/psi or more and total rejection of 80% or more. The minimum velocity of flow/retentate through the lumens of the membranes may be between 0.15 ft/s and 0.6 ft/s.
In various other aspects, the invention provides a reverse osmosis or nanofiltration apparatus including a membrane module. The filtration module may have a plurality of preceding or succeeding stages of hollow fibre membranes suspended between opposed headers, as described above. The module has a module feed inlet, a module retentate outlet and a permeate outlet. A feed water passageway fluidly connects the module feed inlet to a source of pressurized water such as a well pump or a municipal water supply, optionally increased in pressure with a supplemental pump. The permeate outlet is preferably connected to a permeate tank such as a diaphragm tank or air cushion tank in which pressure is related to the volume of water in the tank. When a selected pressure in the permeate tank is reached, any feed side pumps are shut off and the module retentate outlet is closed. Preferably, the membranes have a minimum permeability of 0.1 gfd/psi, minimum rejection of 80% and a minimum hardness rejection of 70%. The minimum flow velocity of feed/retentate is preferably between 0.15 and 0.6 ft/s, and more preferably between 0.2 ft/s and 0.3 ft/s. The feed/retentate passes through the module without being recirculated, preferably with an overall module pressure drop between 30 psi and 120 psi.
In other aspects of the invention, processes for cleaning and reducing scale formation on membrane surfaces are described. Particularly when the module is used to provide a softened permeate, carbonate scale may form in the membranes. To control scaling, suitable cleaning chemicals, such as acids or chemicals that produce acids in water, for example carbon dioxide or citric acid, are injected into the feed/retentate side of the module, either dissolved into a liquid such as feed water or, in the case of carbon dioxide, as a gas. A controllable cleaning chemical addition system is operable to inject a fluid comprising cleaning chemical into the pressurized feed water or the feed/retentate side of the module.
In a continuous while permeating method, the cleaning chemical is injected substantially continuously into to the feed water while the apparatus is producing permeate. Where the cleaning chemical is carbon dioxide, the carbon dioxide is preferably injected in amounts such that the Langelier Scaling Index of the feed water is zero or slightly negative. Optionally, carbon dioxide may be injected only into later stages of the module. In a discontinuous while permeating method the cleaning chemical, such as carbon dioxide, is injected into the feed water periodically while the apparatus is producing permeate. In another method, the direction of flow through the module is reversed while the cleaning chemical, such as carbon dioxide, is being added to apply cleaning chemical to the module from what is at other times the retentate outlet.
In a continuous without permeation method, the cleaning chemical, such as carbon dioxide, is injected substantially continuously to the feed water while the apparatus is not producing permeate. A retentate outlet is more fully opened to allow the feed/retentate to flush through the lumen side of the module to a drain. In a hold and flush method, permeate production is also temporarily stopped and the retentate outlet is more fully opened. A fluid containing cleaning chemical, such as carbon dioxide, flows into the module inlet, accomplished for example by injecting compressed carbon dioxide gas into a flow of feed water flushing through the lumen side of the module inlet. The fluid containing cleaning chemical displaces the feed/retentate in the lumens of the membranes until a substantial part, and preferably all, of the volume of the lumens of the hollow fibre membranes contains the cleaning chemical. The flow of the fluid containing cleaning chemical is stopped and the cleaning chemical is permitted to react with foulants for a selected hold time. Optionally, the module may then be flushed with feed. The selected hold time is typically between 1 and 30 minutes or between about 10 minutes and 20 minutes. The flush and hold method is performed periodically, for example once a day during a time when demand for permeate is low.
In a gaseous cleaning method, carbon dioxide gas enters the feed/retentate side of the module and displaces the feed/retentate. The gas is held in the module under pressure for a period of time and then flushed out with feed water. For additional cleaning, the process may be repeated.
All references to gallons in this applications refer to US gallons.